ADAU1761 produces "modem like sounds"
Hi,
An ADAU1761 based product is entering pre-production. The product designers have highlighted an area of concern. They'd like to understand what they are hearing. To hear:
Load the attached project to the ADAU1761 evaluation board. Turn the volume slider to full. For the LINP/LINN pair: Toggle Mixer enable to "enabled". Toggle Differential path to "disable". "Mute" left volume control. By toggling these two: Mixer Enable and Differential Path, with no audio inputs, we can hear modem like sounds. They are very apparant using a decent set of headphones. Once the sounds appear they will continue for Mixer Enable "enabled", Left Vol Control "Mute" and Differential Path "Disable" settings. There are other conditions that produce the modem lke sounds as well and can be found with a little experimentation.
The customer would like to know where these are coming from and what if anything can be done about it.
Thanks,
Phil
Audio Signal Processors
SigmaDSP Audio Processors
adau1761
Hi Phil,
I am able to replicate your issue on the ADAU1761 evaluation board. I suspect there are potential issues with the eval board Analog Input circuitry and layout. A few observations:
1) I can also hear these modem sounds. If I unplug the stereo audio cable from the IN3/4 source on the 1761 evaluation board, or from the PC stereo output jack, the modem sounds are significantly reduced, but still audible.
2) I also notice that certain I2C accesses will generate audible clicks with the mixer enabled. For example, moving the Left Control and Right Control differential path sliders for the Record Input Signal Path will generate these clicks. Same is also true when making slider adjustments for the ALC, digital ADC/DAC volume or playback volume signal path sliders. Seems to correlate to the Block Write accesses in the Capture window.
3) If I disable the ADC mixer and route the audio through the playback left/write auxiliary bypass, I can still hear the noise. But then, muting this bypass path, and moving any register sliders, the modem noise and audible clicks go away as well. So the source is definitely "analog" and nothing to do with the ADCs or digital path on the chip.
Does the customer hear these same issues on their own hardware? I will look into this further and get back with you on what could be causing this.
Regards,
-JT
Hi John,
Thanks for spending some time with this. I confirm a number of your observations. Yes, the noise shows up on their target hardware units.
Thanks,
Phil
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